Liquid crystals currently are used in a wide variety of devices, including optical devices such as visual displays. Such devices usually require relatively low power, have a satisfactory response time, and are relatively economical. The property of liquid crystals enabling use, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter and/or to absorb light, on the other, depending on the alignment (or lack of alignment) of the liquid crystal structure, for example with respect to an electric field applied across the liquid crystal material. An example of electrically responsive light crystal material and use thereof is provided in U.S. Pat. No. 3,322,485.
Certain liquid crystal material is responsive to temperature, changing optical characteristics in response to temperature of the liquid crystal material. The invention of the present application is disclosed hereinafter particularly with reference to the use of liquid crystal material that is particularly responsive to an electric field.
Currently there are three categories of liquid crystal materials, namely cholesteric, nematic and smectic types. The invention of the present application relates in the preferred embodiment described below to use of liquid crystal material which is operationally nematic (described hereinafter). However, various principles of the invention may be employed with various one or ones of the other known types of liquid crystal material or combinations thereof. The various characteristics of the cholesteric, nematic and smectic types of liquid crystal material are described in the prior art.
One characteristic of nematic liquid crystal material is that of reversibility. Cholesteric material is not reversible. A characteristic of reversibility, in turn, is that the liquid crystal structure will return to its original configuration after an electric field has been applied and removed.
To enhance contrast and possibly other properties of liquid crystal material, pleochroic dyes have been mixed with the liquid crystal material to form a solution therewith. The molecules of the pleochroic dye generally align with the structure of the liquid crystal material. Therefore, pleochroic dyes will tend to function optically in a manner similar to that of the liquid crystal material in response to a changing parameter, such as application or non-application of an electric field. Examples of the use of pleochroic dyes with liquid crystal material are described in U.S. Pat. Nos. 3,499,702 and 3,551,026. The White et al article in Journal of Applied Physics, Volume 45, No. 11, November, 1974, at pages 4718-4723, mentions the use of cholesteric liquid crystal material added to nematic liquid crystal material together with pleochroic dye to improve contrast ratio of an optical display formed thereof.
A characteristic typical of liquid crystal material is anisotropy. An anisotropic material has different physical properties in different directions. For example, liquid crystals are usually optically anisotropic, i.e. they have indices of refraction which vary with the direction of propagation and polarization of the incident light. Such characteristic of birefringence is utilized in the encapsulated liquid crystal material, for example, disclosed in the above-referenced applications to improve the scattering and/or absorption of light when in the field-off condition. For example, the liquid crystal material has an index of refraction that is quite different from that of the containment or encapsulating medium for absorption in the field-off condition and that substantially matches that of the containment medium in the field-on condition for transmission of light. Due to such birefringence, though, the integrity, clarity, focusing, and the like of an image intended for transmission through the liquid crystal material becomes nearly impossible, especially when the liquid crystal material is not aligned with respect to an electric field, i.e. field-off condition.
Liquid crystal material also has electrical anisotropy. For example, the dielectric constant for nematic liquid crystal material may be one value when the molecules in the liquid crystal structure are parallel to the electric field and may have a different value when the molecules in the liquid crystal structure are aligned perpendicular to an electric field. Since such dielectric value is a function of alignment, for example, reference to the same as a "dielectric coefficient" may be more apt than the usual "dielectric constant" label. Similar properties are true for other types of liquid crystals.
A discussion of the encapsulation of operationally nematic liquid crystal material is presented in my above copending applications. Some brief discussion of the encapsulation of cholesteric liquid crystal material is presented in U.S. Pat. Nos. 3,720,623; 3,341,466; and 2,800,457, the latter two patents being referred to in the first.
The advantages inuring to use of encapsulated liquid crystal material, especially of the operationally nematic type, for relatively large scale optical displays and light control devices are described in my above copending applications. Several characteristics of such encapsulated liquid crystal material for such use include the effective eliminating of the fluid nature of the material because the liquid crystal material would be contained in discrete or at least relatively discrete containment volumes, such as capsule-like spheres formed in an emulsion with a containment medium. The capsules could be applied uniformly, e.g. at a uniform layer thickness, on a support medium so that the optical and electrical characteristics of the resulting device will be correspondingly uniform, for example. Moreover, if desired, the capsules could be applied only where needed, thus saving on the amount of liquid crystal material required for the given device.